[GRPC] Handling of pointers and optional arguments

[vaastav]

In GitLab by @JonathanMace on Sep 16, 2023, 24:31

The current GRPC plugin implementation is a 'golden case' implementation that only handles structs and questionably handles pointers. It is untested what will happen for methods that take pointer types.

Implementing support for pointers is relatively straightforward:
1 For an RPC method argument, a pointer type should translate into an optional GRPC message field.
2 In the marshall and demarshall code, there should be nil checking for pointer types.